1. Field of the Invention
The present invention relates generally to a method for controlling combustion condition in a combustion apparatus such as a boiler or an industrial furnace.
2. Description of the Related Art
A boiler generates steam by heating up water with a burner, and supplies the steam to an equipment such as a heating device. In a system including the boiler and heating device, the steam pressure of the boiler will vary according to the amount of the steam which is consumed by the heating device. Therefore, the operating condition of the boiler should be controlled to maintain the steam pressure constant.
A conventional controller for a boiler includes a valve for controlling the flow of fuel, which is disposed along a pipe for feeding the fuel to the burner, and a valve for controlling the throughput of air, which is disposed along a pipe for feeding the air to the burner. To control the fuel flow to the burner, the controller controls an opening angle of the fuel control valve, via a control motor, so that the steam pressure detected by a pressure sensor approaches a predetermined pressure level. Further, the fuel control valve is connected to the air control valve, via a mechanism such as a link motion, to control the throughput of air in accordance with the fuel flow control. Accordingly, an actuation of the single control motor causes the fuel and air control valves to be simultaneously controlled.
However, it is impossible to achieve precise control of the throughput of air using the conventional controller. Because, the conventional controller is designed to just control the angle of the fuel control valve, and the angle control of the air control valve is therefore considered as a secondary control. In order to avoid air deficiency in any circumstances, the air control valve must be designed in advance to permit the air exceeding a theoretically proper amount to be supplied. Consequently, while the boiler is operating, the excess air supplied to the burner takes the boiler's heat away, and discharges the heat through a high temperature exhaust gas. In other words, the excess air feed reduces the thermal efficiency of the boiler. Such situation is not conducive to achieving high energy efficiency.
To solve the foregoing shortcomings, Japanese Unexamined Patent Publication 3-294721 discloses a combustion control system which includes a first control motor for controlling an angle of the fuel control valve and a second control motor for controlling an angle of the air control valve. In the control system, the feedback control of the air control valve is executed, independent of the fuel flow control, such that the throughput of air is most preferably controlled according to the fuel flow.
According to the control system, an optical sensor detects the radiated light originated in the combustion flame of the burner, and converts the detected light into a respective electric signal. FIG. 23A shows data relating the electric signal with respect to elapsed time elapsing, at every excess air ratio. The excess air ratio is defined as the ratio of the actual supplied air amount to the theoretical air amount which is required to completely burn a predetermined amount of fuel. The electric signal (i.e., combination signal including various frequencies) transmitted from the optical sensor is processed the well-known frequency analysis. The frequency analysis clarifies the relation between the frequencies (Hz) of each elemental signal of the combination signal and the signal strength (dBV) thereof. FIG. 21B shows the result of the frequency analysis, at every excess air ratio. The signal strength is integrated in the entire analyzed frequency region. This integrated value is referred to as an oscillation power.
In a certain case, the oscillation power, combustion rate and excess air ratio form a following correlation equation (1): EQU .lambda.=c.times.exp (p.times.f(x)) (1)
in which ".lambda." is the excess air ratio, "c" is a constant value, "p" is the oscillation power, and "f(x)" is a function relating to the combustion rate.
According to the equation (1), the excess air ratio (.lambda.) is a monotone increasing or decreasing function with respect to the oscillation power (p), and those elements show a one-to-one correlation. Therefore, the control of the excess air ratio utilizing the function (1) enables efficient combustion control for the combustion apparatus. TOYOTA Tecnnical Review Vol.41 No.2 April 1992 (English Version), Page 42-50 "Study of an Optical Frequency Type Combustion Control Method", written by the inventors of the present invention, describes in detail that the oscillation power calculated through the above-described manner on the basis of the radiated light originated in the burner flame may be utilized as an indicator for excess air ratio control in the combustion apparatus.
This article states the oscillation power as follows:
"The oscillation power as the total sum of turbulence of the turbulent combustion flame was considered the indicator of the intensity of turbulent, and the experimental result suggested that the turbulence is closely related to the combustion state."
However, some types of the combustion apparatuses do not have a one-to-one correlation between the oscillation power and the excess air ratio. It is found that a chart of the correlation has a mountainous shape similar to a negative quadratic function, as shown in FIGS. 25 and 26. This fact suggests that the oscillation power reflects not only the fluctuation of the turbulent combustion flame but also the intensity of radiated light from the combustion flame (or an other factor corresponding to the intensity of radiated light). This point will be described in more detail, referring to an example.
The electric signal corresponding to the radiated light of the combustion flame of the burner, which is detected by an optical sensor, can be divided into a signal element indicative of the intensity of radiated flame light, and a signal element indicative of the oscillation reflecting the fluctuation of the turbulent combustion flame. FIG. 24 shows the relation between the excess air ratio and the signal element of light intensity. FIGS. 28A and 28B show the changes in the respective signal elements of light intensity and oscillation with respect to elapsed time. Furthermore, FIGS. 25, 26 and 27 show the correlations between the excess air ratio and the oscillation power, when the set frequencies for the frequency analysis are 20 Hz, 50 Hz and 300 Hz, respectively.
A comparison of FIGS. 24 and 25, shows there is strong correlation between the oscillation power and the signal element of light intensity, in the case that the set frequency is a relative low frequency such as 20 Hz, where the fluctuation of the turbulent combustion flame is less influenced by the change of the air throughput. The oscillation power is strongly influenced by the intensity of radiated light.
As shown in FIG. 23B, the higher the excess air ratio becomes, the greater the signal strength in the high frequency region becomes, due to the influence originated in the fluctuation of the turbulent combustion flame. Accordingly, when the set frequency for the frequency analysis is set to a rather high value (e.g., 300 Hz), the signal strength in the high frequency region increases as the excess air ratio increases. Consequently, the peak value of the oscillation power (i.e., the summit of the negative quadratic function) is shifted to the high excess air ratio side.
On the contrary, when the set frequency for the frequency analysis is set to generally small value (e.g., 20 Hz to 50 Hz), the peak value of the oscillation power is shifted to the low excess air ratio side as shown in FIGS. 25 and 26. It should be noticed that the peak value of the oscillation power in FIG. 25 (at 20 Hz of set frequency) is located in the lower excess air ratio side, comparing with that in FIG. 26 (at 50 Hz of set frequency which is slightly higher value than that in FIG. 25). The correlation between the excess air ratio and the signal element of light intensity, corresponding to the condition in FIG. 26, may be generally similar to that as shown in FIG. 24.
The reason for the occurrence of these phenomena originates in the proportional change of the amplitude of the oscillation signal element with respect to the change in the intensity of radiated light, that is understandable through the comparison between FIGS. 28A and 28B. As a normal furnace of the combustion apparatus is adiabatic to some degree, the internal temperature of the furnace is increased by the combustion of the fuel and air. The rise of the internal temperature increases the light intensity of infrared rays, which is detected by the optical sensor. As a result, the signal element of light intensity corresponding to the intensity of the detected infrared rays increases, and the amplitude of the oscillation signal element is increased proportionally with respect to the intensity of light.
The signal element which is strongly influenced by the intensity of light is particularly the element having a large amplitude (i.e., low frequency signal). Therefore, when the set frequency for the frequency analysis is set to a low value, the calculation of the oscillation power is greatly influenced by the signal element of light intensity rather than the oscillation signal element. Thus, the characteristic of the oscillation power is coincident with the characteristic of the signal element of light intensity as shown in FIG. 24.
According to the mountainous shaped charts as shown in FIGS. 25 and 26, even when the value of the oscillation power is specified, two solutions (i.e., two excess air ratios) corresponding to the specified oscillation power may exist in the limited range of excess air ratio to be utilized for combustion control. In this case, the oscillation power can not be an indicator of the excess air ratio control. The above-described equation (1) is just effective in a specific limited region of the excess air ratio. Accordingly, the application of the control method for the excess air ratio based on the oscillation power is just limited to some types of the combustion apparatuses.
To improve the practical use of the new method for excess air ratio control, it may be proposed to set the maximum measuring frequency for the frequency analysis to a high value. When the set frequency is 300 Hz as shown in FIG. 27, the oscillation power generally corresponds to the excess air ratio in the one-to-one manner. Then, the excess air ratio control based on the oscillation power can be achieved.
Even in the proposal, however, it has not been solved yet that the influence originated in the intensity of light causes the chart indicating the correlation between the excess air ratio and the oscillation power to become a mountainous shape. Accordingly, even when the frequency analysis in the wide frequency region including the very high frequency region (e.g., several hundreds Hertz through several thousands Hertz) is always carried out, the mountainous characteristic may be still maintained in response to the type of the combustion apparatus or the kind of fuel. Therefore, the conventional method for controlling the excess air ratio based on the oscillation power as an indicator has no wide use.